Ventura County UCCE Farm Advisor, Ben Faber gives advice on how to identify and treat root rot in avocados.
How to identify root rot and treat it?
The canopy is thinning. The leaves are small and yellow. There is dieback in the canopy, with leafless tips on the branches. You dig around under the canopy in the wetted area of the sprinkler and you can’t find roots within 6 inches of the soil surface or if you do find them, they are black. There is little mulch under the tree. There are weeds growing under the tree. All these are signs of root rot disease. But it is also a sign of lack of water, because that is what is happening – there are no roots to take up water. At this point, gardeners may unfortunately water the tree more, which only makes conditions worse for a diseased tree; adding more water to a tree that can not easily take it up, creates asphyxiation conditions.
So what do you do if you have disease? First of all, make sure you are irrigating to the needs of the tree by checking soil moisture before irrigating and ensuring the tree is not receiving supplemental water from another area such as a lawn sprinkler. Add and maintain a good thick layer of woody mulch under the canopy. Adding gypsum (15-20 pounds per tree), evenly spread under the canopy, can also help. There are fungicides available from retail nurseries, but reviewing and modifying irrigation practices and maintaining a mulch layer are the two most important things you can do.
Read on for details on mulch and irrigation.
Mulch and avocados?
“I just raked up all the leaves under the avocado and it looks so nice,” you say. PUT THEM RIGHT BACK. The avocado is shallow-rooted and really depends on the natural leaf mulch to protect its roots. In fact, the roots will actually colonize the rotted leaves as if it were soil. This mulch is also a first line of defense against root rot. The decomposing leaves create a hostile environment to the microorganism that causes the disease. The mulch also helps to reduce evaporative loss of water and therefore reduces water needs. Commercial growers will actually spread mulch in cases where trees are too young to produce adequate leaf drop for mulch or in windy areas where mulch has blown away. The key to remember is that the mulch should be kept at least 6 inches away from the trunk to avoid collar rot, which can be caused by keeping a moist mulch against the trunk.
How to irrigate avocados
Mature avocado trees may be large above ground, but they have very shallow roots mostly in the top 8 inches of soil. The tree therefore does not have access to a large volume of stored water. As opposed to a deep-rooted walnut, avocados require frequent, small amounts of water. A young tree in the summer might need multiple applications per week, but because the root system is small, each application may only be 5-20 gallons. An older tree with its wider rooting pattern may go a week to a month between irrigations depending on the weather and rainfall. Proper irrigation is the best way to keep the avocado from getting root rot. Both over and under irrigation can induce the conditions for root rot, although over irrigation is more common. And remember, it is not just the amount applied at an irrigation, but the timing that is important, as well. Because you are managing such a shallow root system, just poking your finger into the root system will tell you if there is adequate moisture there before you irrigate again.